Fabrication of co-cr-al-y feed stock

ABSTRACT

The process of making an ingot for use as a feed stock for a coating process in which the ingot is vaporized to produce the coating material; the ingot being made by forming a billet of the alloy minus the reactive element with a rod of the reactive element of the alloy extending centrally through the billet and then shaping the billet to ingot size, the cross-sectional area of the rod having the same relation of the remainder of the area of the billet as the percentage of the reactive element in the alloy.

SUMMARY OF THE INVENTION

For adequate protection of some alloys exposed to heat and corrosivegases, such as some of the high temperature alloys in gas turbines, aprotective coating for the alloys has been found to providesignificantly longer life for the turbine parts. This is particularlytrue of turbine blades and vanes. Such a coating is regularly applied byvapor deposition of the coating alloy, the vapor produced as by electronbeam vaporization, in a vacuum, of an ingot made of the coating alloy asdescribed, for example, in U.S. Pat. No. Bleckerman et al 3,620,815 orBala et al 3,667,421. One problem has been the preparation of the ingotto be evaporated, especially when one of the elements of the alloy isreactive such as yttrium, hafnium or other elements of this type. Thecoating, to be most effective, must have such reactive element orelements present in a precise percentage and thus the ingot must havethe proper amount of this reactive element.

To obtain the desired composition the ingots are presently produced byinvestment casting the desired alloy and then machining the casting toingot size. Such techniques are much more expensive than casting largebillets and subsequent extrusion to size. With certain alloys, however,the presence of second phases associated with the reactive element ofthe alloy makes this latter technique impractical.

Further, some of these alloys with the reactive element cast in are verydifficult to extrude into ingot size and shape for use in vapordeposition. Also in making the casting, the reactive element promotesdecomposition of the crucible material by reacting with it, frequentlyresulting in impurities in the casting and loss of the desired quantityof the element in the finished casting. Additional quantities of thereactive element in the crucible added during melting to compensate forthe lost quantity of the element merely increase the detrimentaleffects. The reactive element, particularly yttrium also tends tosegregate in the casting and can produce compositional variations in thevapor during the coating process.

The presence of impurities in the ingot, as from the decomposition ofthe crucible during casting, can cause eruptions in the pool of moltenalloy during vapor deposition, thereby introducing globules of moltenpool alloy on the coated part. These particles can cause rejection ofthe finished part. Although the coating on the rejected part may bestripped and the part recoated, it is an expensive process and greatlyincreases the cost of the finished part.

The present invention avoids these problems by forming a large billet asby casting the alloy minus the reactive element, placing the reactiveelement in a form of a wire or rod in an axially extending hole in thecasting, and then extruding the billet to the desired dimension for theingot. The diameter of the rod or wire of the reactive element providesa cross-sectional area that has the same relation to the area of thebillet as the proportion of the reactive element in the alloy. Theresulting ingot thus has the composition required in the coating alloy.Although the reactive element is desirably located centrally of theingot it is not necessarily so located so long as it extends axially ofthe ingot and with the proper cross-sectional area.

The billet may have a cored hole to receive a rod of the reactiveelement. Alternatively, certain alloys which are machinable without thereactive element present may have the center hole drilled or the billetmay be split, a groove machined in the matching faces, and the halves ofthe billet then put back together around the central rod.

The ingot to be used in vapor deposition may be made by other techniquesas by forming powdered alloy into ingot shape by hot isostatic pressing.In this event, the powdered alloy, minus the reactive element, isencapsulated in a container of a material not reactive to the alloy andcapable of withstanding the temperatures and pressures of the heatingand pressing treatment necessary in forming the ingot. In this techniquethe rod of reactive metal is positioned centrally of the powdered alloyin the capsule and is thus centrally located during the forming of theingot to shape.

The result is a simplified procedure for producing a usable ingot forvapor deposition in which the reactive element is separate from the restof the metals of the alloy during the forming of the ingot to finishedshape thereby avoiding both loss of this element and undesirable secondphases in the alloy and precise control of the percentage of thereactive element in the ingot. With the desired percentage of thereactive element in the ingot, the same percentage will be in thecoating vapor and in the coating on the coated article. The ingot of theinvention also assures a greater homogeneity in the coating and theproper quantity of the reactive element throughout the length of theingot.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in the light of the followingdetailed description of preferred embodiments thereof as illustrated inthe accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a side elevation of the finished ingot.

FIG. 2 is a sectional view along line 2--2 of FIG. 1.

FIG. 3 is a sectional view of another ingot.

FIG. 4 is a schematic view of the coating process using this ingot.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The ingot of the present invention is shown in FIGS. 1 and 2 asrectangular in cross section to fit the crucible into which the ingot isfed. Such crucibles as well as the coating technique are well known asdescribed in U.S. Pat. No. to Bala et al 3,667,421 above mentioned, inwhich the ingot is fed upwardly into a bottom-feed crucible, is meltedby an electron beam to provide an alloy vapor in a vacuum chamber inwhich the piece or pieces to be coated are positioned.

Thus the ingot 10, shown as rectangular in cross section, but which maybe any other cross-sectional shape to fit the crucible, has a centrallylocated rod or wire 12 of the reactive element of the alloy. This wire12 extends the entire length of the ingot and has a cross-sectional areahaving a ratio to the remainder of the area of the ingot comparable tothe percentage of the reactive element in the alloy. One of the alloysin regular use is a chromium, cobalt, and aluminum alloy to whichyttrium is added as the reactive element. This alloy is well known andis in use and is a part of the invention only as it relates to theparticular composition of the ingot and to the process for making theingot. Other alloys to which invention is applicable are alloys ofeither iron or nickel, or both nickel and cobalt or iron and cobalt withchromium and aluminum. These alloys are well known; the use of areactive element such as yttrium improves the durability and performanceof the vapor deposited coating using these alloys.

The invention process will be described with respect to the alloy knownas Co-Cr-Al-Y. Obviously, other alloys and other reactive elements couldwell be used in producing the ingot of this invention; the essentialfeature to enclose the reactive element within the ingot so the reactiveelement will be present in the proper proportion in the ingot withoutbeing mixed with the other metals of the alloy of the ingot. The ingotis made by first casting a large billet of chromium, cobalt, andaluminum in the proper proportions to produce the alloy minus thereactive element. This billet 10 has a hole 14 therein extending axiallythe entire length of the billet and preferably centrally of the billet.The wire or rod 12 of the reactive element yttrium is positioned withinthis hole, preferably substantially filling the opening. This rod orwire may be formed by any well-known process, as by rolling or drawing,yttrium by itself being quite ductile. The area of the hole, and thusthe wire therein, has the same relation to the total area of the billetas the portion of the reactive element to the total alloy. Thus for anycross section of the billet the chemical analysis would be that of thecoating alloy.

The hole in the casting may be made in any of several ways. It may beformed by a core positioned in the mold during casting of the billet,and this is particularly desirable when the alloy of the billet is notreadily machinable. Alternatively, if the billet alloy can be machinedto some extent, the billet may be cast solid and then split lengthwise.A groove is then milled or otherwise cut in each of the halves of theingot and the halves are then assembled around the reactive element. Formore readily machinable alloys, the ingot may be drilled to receive therod.

The billet with the rod of the reactive element therein is then extrudedto produce an ingot of the desired shape and dimension for the crucible.Because the alloy without the reactive element is more workable, it ispossible to extrude the two-piece billet to ingot dimension, thereactive element being generally readily worked by extrusion. During theextrusion the reactive element is protected from oxidation or othercontamination by the surrounding alloy and the result is a usable ingotwith the proper quantity of reactive element at all sections of theingot. When the ingot is then fed into the crucible, the resultingcoating vapors will have the desired composition for most effectiveprotection of the coated article.

The reactive element is shown and described as being centrally locatedwithin the ingot. This is an advantage in melting by electron beam inthe crucible and in vaporizing the alloy in that the concentration ofthe beam is greatest centrally of the crucible. It has been found thatthe convective currents developing in the molten alloy will assuremixing of the reactive element with the rest of the alloy. Where therequirements of the quality of the coating applied by the vaporizationof the alloy are less stringent, it may be acceptable to use an ingot inwhich the reactive element is inserted in a groove milled or otherwiseformed in the wall of the billet before extrusion. In this event, theratios of the cross-sectional area will be maintained as abovedescribed. Such a billet and the resulting ingot can be produced asreadily as the billet with a reactive element centrally of the billetand is equally readily extruded to ingot shape and dimension.

Although the billet is described above as cast it may be produced inother ways as by making an encapsulated billet from the alloy inpowdered form minus the reactive element, with a wire or rod of thereactive element then positioned in the encapsulated powder, preferablycentrally, the rod extending axially of the encapsulated powder from endto end. In making an ingot by this technique, the powdered alloy isplaced in a capsule of a material that is not reactive with the alloyand that will withstand the pressures and temperatures of the hotisostatic pressure technique used in making the ingot. Glass or metalmay be used for this purpose, although other material may be usable. Thecapsule is filled with the powdered alloy with the reactive rodpositioned therein either before or after the powdered alloy is in thecapsule. In either case, the rod is located within a passage in thepowdered alloy, the rod forming the passage in the alloy either as thepowdered alloy is placed around it or by forcing the powdered alloyaside if the rod is inserted into the filled capsule. After filling, thecapsule is evacuated and sealed so that the powder is in a vacuum.

The cross-sectional area of the rod in this case will have the sameproportion to the compacted area of the surrounding alloy as thepercentage of the reactive element in the alloy. Since the rod isalready in solid form it is not significantly compacted during thesubsequent heating and pressing of the material and is not significantlychanged in dimension. In making the ingot in this way there is noextrusion of the material as such and the finished ingot is the samelength as the length of the powdered alloy in the capsule.

The capsule, with the rod and powdered metal sealed therein in a vacuumis then formed to size by hot isostatic pressure techniques. To do this,the capsule is placed in an autoclave and heated under such a pressurethat the powder will be compacted into solid form, with the eliminationof most of the voids therein. The dimension of the capsule is selectedso that when the powdered alloy is compacted to the desired extent itwill have reached the desired ingot dimension in cross section. Oneadvantage of this technique is that the finished ingot cross section maybe controlled accurately enough to assure a proper fit within the bottomopening of the vaporizing crucible without a need for further shapingthe outer surface of the ingot to more precise dimension.

An ingot made in this way may have the configuration of FIG. 3. In thisfigure, the reactive rod 16 is enclosed within the surrounding compactedalloy material 18 to form the complete ingot 20. The outer periphery ofthe ingot is shown as circular to fit in a circular bottom opening inthe melting crucible used in the coating process. This ingot, as abovestated, has a rod cross section related to the cross section of theremainder of the ingot to correspond to the percentage of the reactiveelement in the coating alloy. When this ingot is completed, the ingothas the reactive element so enclosed that it will produce the properquantity of this element as the ingot is melted for the coatingoperation.

After the ingot is prepared as above described, it is used in a vaporcoating process as shown schematically in FIG. 4. As shown, the ingot 20is fed upwardly through a crucible 22 which is preferably water cooledas shown. A suitable mechanism feeds the ingot vertically. The topsurface of the ingot is exposed to an electron beam 24 from an electronbeam gun 26. This melts the surface of the ingot to form a pool 28 ofalloy from which a vapor cloud of alloy is evaporated and this couldenvelops the workpieces 30 that are to be coated. The process is carriedout in a vacuum chamber 32. Because the relationship of the core or rodof reactive material in the ingot to the surrounding alloy is properlydetermined, the resulting cloud of vapor surrounding the workpieces,shown as turbine blades, has the desired composition to produce the mosteffective coating on these workpieces. The details of this coatingprocess are well known. The importance of the present invention is thatit provides a vapor cloud of the correct chemistry to deposit anacceptable coating on the workpiece without any impurities resultingfrom reaction of the reactive element with the crucible, in the billetmaking process thereby eliminating objectionable deposits from anyeruptions from the molten pool.

Although the invention has been described with respect to Co-Cr-Al-Y itshould be understood that it is equally applicable to any other alloyhaving a reactive element where the reactive element is ductile andwhere the alloy minus the reactive element is castable and adequatelyductile for extrusion of the composite billet.

Having thus described a typical embodiment of our invention, that whichwe claim as new and desire to secure by Letters Patent of the UnitedStates is:
 1. In the production of an ingot of a selectedcross-sectional dimension for use in crucible vaporization for a coatingprocess, the ingot being an alloy having a reactive element as oneconstituent, the alloy without the reactive element being workable andthe reactive element being workable, the complete alloy with thereactive element therein however being much less workable, the completedingot being fed axially through an opening in the crucible forvaporization to produce a coating vapor having the elements of the alloyin the desired proportions including the reactive element, the cruciblehaving an opening to receive the ingot, the steps of:forming anelongated billet having the ingredients of and in the proportions of thealloy and without the reactive element included therein; making a holefrom end to end of the ingot, this hole having an area related to thearea of the billet substantially the same as the percentage of reactiveelement in the alloy; positioning a rod of the reactive element withinthe billet, said rod having a cross-sectional area related to the areaof the billet the same as the proportion of the reactive element in thealloy, said rod extending axially of the billet from end to end;reducing the billet from its formed size to an ingot having a shape anddimension to fit the opening in the crucible so that it may be fedtherethrough and the cross section of the ingot having all theingredients of the alloy in proper proportion therein, the ratio of thecross-sectional area of the rod in the ingot to the cross-sectional areaof the ingot being comparable to the desired percent of the reactiveelement in the remainder of the alloy; and feeding the completed ingotthrough said opening in the crucible for vaporization into an alloy. 2.The process of claim 1 in which the reactive element is yttrium and thealloy includes chromium and aluminum with one or more of iron, nickeland cobalt.
 3. In the production of an ingot for use in a coatingprocess in which the ingot is vaporized from a crucible to form a vaporfor coating, the ingot being an alloy one element of which is reactive,and in which the alloy is not readily workable whereas the reactiveelement alone is workable and the alloy minus the reactive element isworkable, the steps of:forming an elongated billet of the ingredients ofthe alloy minus the reactive element and in the proportions of theseingredients in the alloy; positioning a rod of the reactive elementwithin the billet; forming the billet to the shape and dimension of aningot to fit in the crucible, the ratio of the cross-sectional area ofthe rod to the cross-sectional area of the ingot being comparable to thepercentage of reactive element in the alloy; and melting the ingot inthe crucible to form a vapor having the ingredients of the alloy inrequired proportions to form a coating.
 4. The process of claim 3 inwhich the billet is formed by encapsulating powdered alloy, the rod ispositioned in the encapsulated alloy and the billet is then reduced toingot dimension by hot isostatic pressure technique.
 5. The process ofclaim 3 with the additional step of forming an axial opening through thebillet from end to receive the rod of the reactive element.
 6. Theprocess of claim 5 in which the opening is located centrally of thebillet.
 7. The process of claim 3 in which the billet with the rodtherein is formed by extrusion to ingot shape and dimension.
 8. Theprocess of claim 3 in which the alloy includes chromium, aluminum, andyttrium, with one or more of iron, nickel and cobalt therein.